This invention relates to rare earth-containing compound particles for thermal spraying, and a sprayed component. More particularly, it relates to rare earth-containing compound particles which can be plasma or otherwise sprayed to surfaces of metal or ceramic substrates to form highly adherent, smooth and highly pure coatings, and a sprayed component having a coating of the rare earth-containing compound particles.
It is a common practice in the art to thermally spray metal oxide particles to metal, ceramic and other substrates to form a coating thereof for imparting heat resistance, abrasion resistance and corrosion resistance.
The particle powders suitable for thermal spray coatings typically include (1) a fused and ground powder obtained by melting a starting material in an electric furnace, cooling the melt for solidification, and pulverizing the solid in a grinding machine into particles, followed by classification for particle size adjustment; (2) a sintered and ground powder obtained by firing a raw material, and pulverizing the sintered material in a grinding machine into particles, followed by classification for particle size adjustment; and (3) a granulated powder obtained by adding a raw material powder to an organic binder to form a slurry, atomizing the slurry through a spray drying granulator, and firing the particles, optionally followed by classification for particle size adjustment. Although components using alumina, silica or the like have been developed as the sprayed component, it is difficult to produce particle-free dense components.
The thermal spraying particles have to meet the requirements that (i) they can be consistently fed at a quantitative rate to the plasma or flame during spraying, (ii) their shape remains undisrupted during the feed and spraying, and (iii) they are fully melted during spraying. These requirements are quantitatively expressed by more than ten physical parameters of particles.
Since the thermal spraying particles are fed to the spray gun through a narrow flowpath such as a transportation tube, whether they can be consistently fed at a quantitative rate without sticking is largely affected by the powder physical properties and fluidity thereof.
However, the fused or sintered and ground powder resulting from method (1) or (2), though having sufficient strength, has irregular shapes and a broad particle size distribution so that the friction between particles during transportation entails formation of finer particles. Additionally, the powder has a large angle of repose and poor fluidity so that the transportation tube or spray gun can be clogged, preventing continuous thermal spraying operation.
Developed as a solution to these problems of the ground powders was the granulated powder obtained by method (3), that is, having the advantage of smooth fluidity due to the spherical or nearly spherical shape of particles. The strength of the granulated powder tends to vary over a wide range because it depends on the particle size distribution of a raw material powder and the firing conditions. Particles with a low strength will readily collapse during the feed to the spray gun. As the process to make the granulated powder is complicated and composed of number of steps, it becomes difficult to prevent introduction of impurities such as Fe.
In the thermal spraying of metal oxide particles, the particles must be completely melted in the flame or plasma in order to form a sprayed coating having a high bond strength. In the event where granulated powder is prepared using a spray drying granulator, however, an average particle diameter of less than 20 xcexcm is difficult to accomplish. In the event of the fused or sintered and ground powder resulting from method (1) or (2), a spray material having a small average particle diameter is obtainable owing to grinding in a mill, which can cause contamination. When particles are prepared in a conventional way, it is difficult to avoid the introduction of impurities at a level of several ten ppm.
As mentioned above, the fused/ground powder, sintered/ground powder and granulated powder discussed above individually have advantages and disadvantages and are not necessarily optimum as the spray particles. Additionally, the powders of these three types all suffer contamination from the grinding, granulating and classifying steps, which is deemed problematic from the high purity standpoint.
Specifically, the fused/ground powder, sintered/ground powder or granulated powder having passed the grinding or granulating and classifying steps contains impurities such as iron group elements, alkali metal elements and alkaline earth metal elements, typically in a content of more than 20 ppm, calculated as oxide. A sprayed component having a coating obtained by spraying any of these powders is susceptible to corrosion at impurity sites in the coating, failing to provide satisfactory durability.
An object of the invention is to provide thermal spray rare earth-containing compound particles of high purity which can be thermally sprayed to form an adherent coating despite the high melting point of the rare earth-containing compound. Another object of the invention is to provide a sprayed component having the particles spray coated on a substrate surface.
The invention addresses rare earth-containing compound particles for thermal spraying. We have found that by controlling the average particle diameter, dispersion index and aspect ratio to specific ranges, conforming the particle shape to a polyhedron, and optionally, controlling the surface area, bulk density, crystallite size and impurity content to specific ranges, the rare earth-containing compound particles are improved in fluidity and given so high density and strength that the particles are completely melted rather than being collapsed during thermal spraying. A coating obtained by thermally spraying the particles is smooth and pure as compared with conventional sprayed coatings, and offers better adhesion and corrosion resistance.
In a first aspect, the invention provides rare earth-containing compound particles for thermal spraying which are of polyhedral shape and have an average particle diameter of 3 to 100 xcexcm, a dispersion index of up to 0.5, and an aspect ratio of up to 2. Preferably the particles have a specific surface area of up to 8.0 m2/g and/or a bulk density which is at least 0.3 times the true density. Preferably, crystallites have a size of at least 25 nm. The content of each of iron group elements, alkali metal elements and alkaline earth metal elements is preferably up to 5 ppm, calculated as oxide.
In a second aspect, the invention provides a sprayed component comprising a substrate having a surface and a coating of the rare earth-containing compound particles thermally sprayed on the substrate surface.